In the drilling and operation of an oil or gas well, it is frequently necessary to have the production tubing penetrate a layer of a formation which contains high pressure gases or other fluids. This occurs when the desired production layer is below the aforementioned layer containing the high pressure fluids. In such a case, the desired product is brought to the surface through production tubing which must successfully withstand the environment found in the intervening high pressure layer. This environment frequently includes the impingement of high pressure jets of fluid on the outer surface of the production tubing. The high pressure fluid can also contain entrained particulate matter which can be highly abrasive, such as sand.
Operators commonly use sections of production tubing called blast joints to penetrate these layers, because the blast joints are less susceptible to erosion damage than common steel production tubing. A common method of limiting susceptibility to erosion damage is to incorporate into the blast joint a stack of erosion-resistant rings made of such materials as tungsten carbide. This stack of rings is arranged continuously along the outside surface of the production tube to protect it, because the tungsten carbide can resist impingement of abrasives at very high pressures and velocities.
One disadvantage of using carbide rings is that they tend to be very brittle, and they are therefore very susceptible to surface disintegration or even breakage if placed under concentrated mechanical loads such as those imposed by pipe slips or power tongs. This disadvantage is especially serious when the operator wishes to insert the blast joint through a snubber unit.
A snubber unit is mounted on the wellhead of a well which is to be reworked under pressure. Use of the snubber unit allows reworking of a pressurized well without first plugging or killing the well. Killing the well is undesirable because it can be difficult and expensive to resume production from a well that has been killed. A snubber unit establishes and maintains a pressure seal around the tubular goods coming out of or going into the well at the wellhead. It is typically mounted atop one or more blow-out-preventers. In addition to maintaining the pressure seal, the snubber unit grips any tubular goods being inserted and forces them into the well against the wellhead pressure, which can approach several thousand pounds per square inch. Alternatively, the snubber unit can be used to grip a tubular good being extracted from the well to limit or control its outward movement under wellhead pressure.
This gripping of the tubular goods can be accomplished by pipe slips or other devices which place highly concentrated mechanical loads on the goods being gripped. In addition, the pressure seal can only be effective if applied to a relatively smooth surface which is capable of maintaining its pressure integrity under the pressures experienced at the wellhead. It can be seen, then, that a typical carbide blast joint can not be inserted into a well with a snubber unit. If this were attempted, the carbide rings would immediately deteriorate or even fail completely, and the pressure seal could not be maintained. A very expensive and dangerous blowout would occur. For this reason, whenever it has been necessary to insert a blast joint during rework of a well, the well has been killed, rather than using a snubber unit. It would be highly advantageous to design a carbide blast joint which can be used with a snubber installation.